Developers id Software say they have yet to have a team member complete Doom's ultra nightmare difficulty setting. In an interview with IGN, Executive Producer Marty Stratton and Creative Director Hugo Martin said the team has been holding a competition to see who can get the furthest and that no one has come close to completing!

This might be due to the fact that the ultra nightmare setting is not just hard, it has a permadeath feature. That's right, if you die during the hard mode, you have to start the game entirely all over again! The only way you can track your progress is by your singleplayer helmet being present at your furthest point in the game prior.

Sounds like hype to me. We'll see how tough it is when it gets out to a mainstream audience.